Field of the Invention
The present invention relates to a cooling water passage structure of an outboard motor for cooling an engine mounted on the outboard motor by using cooling water.
Description of the Related Art
As shown in FIG. 9, a cooling water passage structure of an outboard motor disclosed in Patent Document 1 (Japanese Patent Laid-Open No. 2013-124592) includes a cooling water passage which causes sea water, river water, or the like taken in through an intake 101 of the outboard motor and led to a water passage 103 by a water pump 102 to flow as cooling water through a cylinder head water jacket 104 of a cylinder head, exhaust passage periphery water jacket 105 of an exhaust passage containing an exhaust purification catalyst, and a cylinder periphery water jacket 106 of a cylinder block in order.
In the cylinder head water jacket 104, a first combustion chamber periphery water jacket 107A and a second combustion chamber periphery water jacket 107B located on an upstream side are communicated with an exhaust manifold periphery water jacket 108 located on a downstream side. The first combustion chamber periphery water jacket 107A, second combustion chamber periphery water jacket 107B, and exhaust manifold periphery water jacket 108 are so designed as to pass cooling water upward from a lower side.
In the case of the cooling water passage structure of an outboard motor described in Patent Document 1, in a peripheral portion of a combustion chamber in the cylinder head, a temperature of a portion in the first combustion chamber periphery water jacket 107A and second combustion chamber periphery water jacket 107B which corresponds to a lowermost cylinder and into which cooling water flows first is lower than that of portions corresponding to the other cylinders, and hence, is overcooled.
Furthermore, in a peripheral portion of an exhaust manifold of the cylinder head, cooling water yet to be cooled in the first combustion chamber periphery water jacket 107A and second combustion chamber periphery water jacket 107B flows into that peripheral portion of the exhaust manifold periphery water jacket 108 which corresponds to the lowermost cylinder, and accordingly, a temperature of such portion is lower than portions corresponding to the other cylinders, and hence, is overcooled.
Consequently, exhaust gas flowing through the exhaust manifold is cooled excessively, steam in the exhaust gas condenses, and droplets are produced in the exhaust manifold.